The invention relates to a reversing valve for a high-viscosity medium, in particular for a plastic melt, comprising at least one housing, having at least one inlet opening and at least two outlet openings. The openings extend into a valve bore in which a valve stem is supported, the valve stem having a recess by which the inlet opening is to be connected alternately with one of the outlet openings.
Such valves are required in systems for the preparation and filtering of plastic melts that are supplied to plastic processing machines. Known are 3/2-way valves designed as rotary valves. The valve stem in this type of valve exhibits a groove at its circumference. The valve stem can be rotated such that:
(a) a flow path is formed between an inlet opening and a first outlet opening via its groove;
(b) a flow path is formed between an inlet opening and a second outlet opening via its groove; and
(c) the inlet opening is closed and all flow paths are interrupted via another surface section on the valve stem into which the groove extends.
It is also known to provide a little longer groove than the length of the wall section between the outlet openings. This results in a transitional position of the valve stem with a respective small opening gap between the groove and the two outlet openings to allow for a pre-flooding of the two paths conducting the melt with a significantly lowered flow rate.
Because of the flow through the groove in all possible positions, no dead zones remain where chemical and thermal degradation of plastics could occur due to long residence times and heating.
However, sealing has proven to be problematic in practical applications. In order avoid leakage flows and at the same time ensure free rotatability, the gap between the valve stem and the housing must be formed very precisely, which is practically achievable only with conical valve stems and conical valve bores. Manufacturing the conical geometries and adhering to the same cone angle for both components requires considerable effort in manufacturing technology.
During operation, the conical valve stem is pre-tensioned to ensure leak-tightness.
An additional disadvantage of the known reversing valve is that the inlet and outlet openings must run radially towards the valve bores. It is therefore not possible to position the two outlet openings without additional reversing members on a common area next to each other.